Technical Abstract:
Desiccation-tolerant plants can be grouped into two categories: 1) desic- cation-tolerant plants whose internal water content rapidly equilibrates to the water potential of the environment and 2) the modified desiccation- tolerant plants that employ mechanisms to retard and control rate of water loss. Desiccation tolerance can be achieved by mechanisms that incor- porate one of two alternatives: cellular protection or cellular recovery (repair). The majority of plants utilize aspects of both. Desiccation- tolerant species, in particular the moss Tortula ruralis, appear to utilize a tolerance strategy that combines a constitutive protection system and a rehydration-inducible recovery mechanism. The rehydration-induced recovery mechanism of ruralis relies heavily upon a change in gene expression that is mediated by post-transcriptional events. Upon rehydration the synthesis of over 80% of the protein complement of gametophytic tissue is altered; either enhanced or depressed. The synthesis of 25 proteins termed hydrins, is terminated, or substantially decreased, and the synthesis of 74 proteins termed rehydrins is initiated, or substantially increased, during the first two hours of hydration. Utilizing cDNAs representing individual hydrins and rehydrins, it was determined that if drying rates were slow rehydrin tran- scripts selectively accumulate in the dried gametophytes. Studies revealed that this storage involves the formation of mRNPs (messenger riboneucleo- protein particles) that are specific to dried tissues. These mRNPs aggregate under low salt conditions into large particles. The identity and possible functions of the rehydrins of Tortula ruralis are under investigation and will be discussed.